This invention relates to an optically functional device. More specifically, the invention relates to an optically functional device having an optical waveguide structure including a corrugation, such as distributed feedback (DFB) laser, in which a change in effective refractive index or a phase shift is made by changing the structure of the corrugation.
Grating-couplers, DFB lasers and DBR (distributed Bragg reflector) lasers are examples of optically functional devices having formed a corrugation along an optical waveguide. In these optically functional devices, the corrugation functions as diffraction gratings, and various functions such as wavelength selectivity, optical feedback, or optical coupling between different waveguides, for example, rely on the corrugation.
Among such corrugations, a corrugation using first-order Bragg diffraction has a period in the order of the wavelength of the guided light in a waveguide. Accordingly, the corrugation is extremely miniaturized, and it is not easy to fabricate it and control its cross-sectional configuration. Average period of the corrugation can be controlled very precisely by holographic interferometry. However, because of the excessively miniaturized pitch, it involved great difficulties in controlling the cross-sectional configuration.
Under the circumstances, it has been avoided to control the function of the waveguide itself by changing the configuration of the corrugation itself. Corrugation is generally regarded as a certain perturbation factor in a waveguide, and corrugation and waveguide are usually separated in function. That is, there was no example in which corrugation relates to a control of the basic function of the waveguide.
For example, in conventional integrated devices of a DFB laser or DFB laser and an electroabsorption modulator (EAM), for example, corrugation was merely provided as a waveguide perturbation. Then, for any change in waveguide parameter along he axial direction, e.g., a change in effective refractive index or optical confinement factor to a given layer, it was necessary to change the layer structure of the waveguide itself.
FIG. 4 is a cross-sectional view illustrating a basic structure of the waveguide of a conventional InGaAsP/InP DFB laser, taken along a plane parallel to the guiding direction of the laser. Electrodes, facet structures for controlling reflection, etc. are omitted from illustration of FIG. 4. The structure shown in FIG. 4 is realized in the process explained below.
First grown on an n-type InP layer (or n-type InP substrate) 101 is an InGaAsP active layer 102 having an energy band gap corresponding to the oscillation wavelength. Although the active layer 102 has a MQW (multi-quantum well) structure alternately stacking InGaAsP thin layers different in composition in most cases, FIG. 4 illustrates it as a single layer for simplicity. Next grown thereon is an InGaAsP guiding layer 103 of a composition transparent to the oscillation wavelength.
Formed on the guiding layer 103 is a first-order corrugation 110. Period of the corrugation must be approximately 200 nm in the wavelength band of 1.3 .mu.m and approximately 240 nm even in the wavelength band of 1.55 .mu.m. To make the corrugation, patterning of a size corresponding to approximately a half of these periods is required. Therefore, an extremely minute processing technique is required, and it is not easy to make it.
In order to ensure an appropriate amount of distributed feedback, depth of the corrugation must be not deeper than 0.05 .mu.m. However, it is not easy to control the depth in this range.
Usually, thickness of the guiding layer 103 is approximately 0.1 .mu.m, and thickness of the active layer 102 is also approximately 0.1 .mu.m. Therefore, in this range of depth of the corrugation, changes in effective refractive index of the entire waveguide are very small. In order to change the effective refractive index of the waveguide in the axial direction, it is necessary to modify the waveguide structure itself by changing the thickness of the active layer 102 and/or the guiding layer 103 themselves or changing their width (the aspect in the width direction is not illustrated), for example.
Next explained is an integrated device of a DFB laser and EAM.
FIG. 5 is a longitudinal cross-sectional view of an integrated device experimentally prepared by the Inventor in the course toward the present invention. Its structure is explained below in sequence of its manufacturing process. First grown sequentially on an n-type InP layer 101 are an active layer 102 and a guiding layer 103 to form the major part of the DFB laser. After that, a first-order corrugation 110 is formed. Then, a portion for the EAM is etched to remove the active layer 102 and the guiding layer 103, and a light absorption layer 106 is grown on the removed portion. The light absorption layer 106 is also a waveguide and in most cases configured as a MQW structure having a band gap slightly larger than that of the active layer 102. When an electric field is applied to this layer, the absorption edge shifts to the longer wavelength side due to QCFK effect (quantum confined Franz-Keldish effect) and/or QCSE effect (quantum confined Stark effect), and the layer becomes absorptive for the output spectral line from the DFB laser portion. The wavelength of the spectral line is determined by the refractive index of the waveguide structure and the period of the corrugation 110. By using this phenomenon and applying a high-speed signal to the EAM portion, output light from the DFB laser under DC operation can be modulated.
For the purpose of electrically insulating the DFB laser portion and the EAM portion from each other, there is provided a region 115 semi-insulated by bombarding protons. FIG. 5 also shows an n-side electrode 130 and p-side electrodes 120 (DFB laser side) and 121 (EAM side).
The structure of FIG. 5 requires, for realization thereof, a process for removing the active layer 102 and the guiding layer 103, once grown, and for newly growing the absorption layer 106. Additionally, this structure involves the problem that, although the optical output from the DFB laser should be smoothly guided to the absorption layer 6, the light becomes discontinuous at the boundary, and part of the light is reflected or scatters.
In the integrated device shown in FIG. 5, these defects might be removed by commonly using a same layer as the active layer 103 and the absorption layer 106.
Fig. 6 is a cross-sectional view showing an improved integrated device.
In the example shown here, the absorption layer 102 and the guiding layer 103 in the DFB laser portion, and the absorption layer in the EAM portion are made of a common layer. Thus, the process of removing layers and the process of growing the absorption layer can be omitted. With the structure of FIG. 6, however, practical performance cannot be obtained actually. This is because the EAM portion also has an active layer, light absorption occurs in this active layer also when no field is applied, and extinction ratio is undesirably reduced. For improving the extinction ratio, it is necessary to reduce confinement of light in the active layer and to increase confinement of light to the guiding layer by increasing the thickness of the guiding layer 103.
However, the approach relying on increasing the thickness of the guiding layer invited problems in the DFB laser portion. That is, undesirable decrease in net gain occurs due to an increase in guiding loss caused by light absorption of the guiding layer 103 and insufficient light confinement to the active layer. There also occurs the problem that it is difficult for the laser portion to oscillate because the pn junction comes remoter from the active layer. Thus, if a layer structure is commonly used in an integrated device, the structure of one of its elements disturbs optimization of the structure of the other element.